At present, 3 GPP provides a concept of Long Term Evolution assisted access (LAA), which is used for an unlicensed frequency spectrum with the help of an Long Term Evolution (LTE) authorization frequency spectrum. When a LTE network uses an unlicensed frequency band, one of the key points is to ensure that the LAA system can coexist with existing access technology (e.g., Wi-Fi) on a fair and friendly basis. A traditional LTE system does not have a listen before talk (LBT) mechanism to avoid collision, in order to better coexist with the Wi-Fi, the LTE system needs an LBT mechanism. In this way, if the LTE detects that a channel on the unlicensed frequency spectrum is busy, the frequency band cannot be occupied. When the channel is detected to be idle, it can be occupied.
As uplink and downlink configuration of traditional time-division duplexing (TDD) is fixed, that is, there are only 7 fixed subframe allocation modes, thus, when the unlicensed frequency spectrum is used in a TDD way, a ratio of time slotting allocations of uplink and downlink is not flexible enough.
In order to better adapt to dynamic changes of uplink and downlink business and the LBT mechanism, for example, when sending data through a Physical Downlink Shared Channel (PDSCH), maximum channel occupation time of the downlink could be 8 ms or 10 ms. If a downlink subframe is converted to an uplink subframe during sending data, a LBT channel detection may be re-executed, this will inevitably lead to the channel being taken away by other systems (e.g., Wi-Fi). Related technologies provide a complete dynamic uplink and downlink subframe configuration, which is frame structure 3, the frame structure 3 is a frame structure used by Secondary cell (Scell) in the LAA system, and each subframe can be changed to an uplink subframe or to a downlink subframe at any time in the frame structure 3.
In this case, there is the a problem that there may be only one uplink subframe or Uplink Pilot Time Slot (UpPTS) or maybe even no uplink subframe or UpPTS in a 10 ms frame. For this case, how to ensure a Sounding Reference Signal (SRS) to be sent normally for ensuring measurement accuracy of uplink channel quality becomes a technical problem to be solved.